1. Field of the Invention
The present invention is directed to a process and apparatus for treating waste water, sewage sludge and/or purifying drinking water via a lime slaking process, and a process and apparatus for dissolving scale that forms on internal surfaces of a lime slaker and related equipment, including an aging tank and a grit separation device, by introducing an acid into the system.
2. Brief Description of the Prior Art
Lime has been used in the treatment of sewage sludge to remove pathogens, lime stabilization of waste water, pasteurization of sludge and purification of drinking water by adjusting the pH of the water. Examples of such processes involving the use of lime can be found in U.S. Pat. Nos. 5,013,458; 5,229,011; 5,346,616; 5,401,402; 5,405,536; 5,433,844; 5,554,279; 5,618,442; and 7,416,673, which contents are incorporated herein by reference.
Calcium oxide (CaO), is mixed with water (H2O) to form calcium hydroxide (Ca(OH)2). The chemical reaction which occurs during such mixing gives off heat in the form of an exothermic reaction. When this reaction is done in the presence of excess water it is commonly referred to as lime slaking. This process is accomplished in a device known as a lime slaker. The resulting mixture of Ca(OH)2 and water is known as a lime slurry. Lime slurries are known to have some unique properties, one of which is an inherent ability to form scale on surfaces which come into contact with the lime slurry. The formation of scale, or “scaling” can render the various delivery systems, such as pipes, troughs, conduits, etc. unusable over time due to the build-up of scale. Accordingly, such scale build-up requires frequent cleaning and descaling of the equipment that is used to handle lime slurries. Cleaning and descaling operations can be considerably labor and cost intensive.
Lime slurries also contain a certain amount of inert material that is commonly referred to as grit. Grit results because the lime contains a certain amount of material other than calcium oxide (CaO). Typically, lime contains approximately 90%-95% CaO, and 5%-10% inert material, or grit. Quicklime is more than 92% pure.
The presence of grit in a lime slurry can cause numerous problems, including grit build-up in downstream process chambers or vessels, grit acting to plug or clog nozzles or any orifices through which the slurry passes, and abrasions caused to treatment equipment and slurry delivery equipment such as pumps, pipes, valves, etc. because of the abrasive nature of grit particles.
Various techniques have been attempted for separation of unwanted grit that is contained within a lime slurry.
U.S. Pat. No. 4,482,528 describes some of the problems that are encountered when trying to separate unwanted grit from a lime slurry. Using a gravity classifier to separate grit from a lime slurry when discharging from a lime slaker is described as being unable to effectively remove particles smaller than 100 mesh (140 microns).
The gravity separation of grit, directly after the slaking of lime in a lime slaking device, can be problematic, in that the amount and size of the grit removed will vary as the rate of discharge from the lime slaker varies. At high discharge rates from the slaker, the slurry will have less retention time in the grit separation chamber, which can be insufficient for the smaller grit particles to settle, such that the grit particles can be conveyed through the separation chamber and into the finished lime slurry, resulting in inconsistent and highly variable amounts of grit removal.
U.S. Pat. No. 4,482,528 attempts to improve upon gravity separation of grit by the use of a cyclone followed by at least one gravity classifier device. The cyclone operates centrifugally, and removes grit and lime, which discharges from the cyclone into a gravity classifier, wherein dilution water is added in order to allow the grit to settle in the classifier.
U.S. Pat. No. 5,746,983 describes an apparatus and method for the slaking of lime which includes a process under constant supervision. The calcium oxide is delivered to the lime slaker and mixed with ambient water through a conveyor which delivers the quicklime at distinct intervals by means of a mixer. The apparatus uses load cells to measure the weight of the lime and/or water. The apparatus and method can be run on small quantities of lime and in batches. This invention is designed to enable non-continuous operation in intervals, and an attempt to limit maintenance on operating systems.
Accordingly, a need remains to provide a method for producing a high-quality supersaturated lime suspension which can be continuously and automatically delivered in precise and variable dosages with little or no supervision and a method for automatically removing scale build-up.